High strength and high modulus carbon fibers

ABSTRACT

A carbon fiber has a fiber tensile strength in a range of 5.5 GPa to 5.83 GPa. The carbon fiber has a fiber tensile modulus in a range of 350 GPa to 375 GPa. The carbon fiber also has an effective diameter in a range of 5.1 μm to 5.2 μm. In a method of making a carbon fiber, PAN (poly(acrylonitrile-co methacrylic acid)) is dissolved into a solvent to form a PAN solution. The PAN solution is extruded through a spinneret, thereby generating at least one precursor fiber. The precursor fiber is passed through a cold gelation medium, thereby causing the precursor fiber to gel. The precursor fiber is drawn to a predetermined draw ratio. The precursor fiber is continuously stabilized to form a stabilized fiber. The stabilized fiber is continuously carbonized thereby generating the carbon fiber. The carbon fiber is wound onto a spool.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/061,327, filed Oct. 8, 2014, the entirety ofwhich is hereby incorporated herein by reference.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under agreement No.W911NF-10-1-0098, awarded by the Army Research Office. This inventionwas also made with government support under agreement No.FA9550-14-1-0194, awarded by the Air Force Office of ScientificResearch. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to carbon fiber technology and, morespecifically to a carbon fiber having both a high tensile strength and ahigh tensile modulus.

2. Description of the Related Art

Polyacrylonitrile (PAN) polymer was commercialized by the DuPont Companyin 1950 and is the only precursor material from which currently highstrength carbon fibers can be processed. These PAN based carbon fibersare now being used for significant structural applications in aerospaceindustry (e.g. in Boeing 787), for power generation (wind mill blades),and are increasingly being used in automobiles. Although carbonfilaments were first used in light bulbs in late nineteenth century,research in modern carbon fibers started in 1959 in Japan and a keyprocessing innovation occurred in England in 1964 resulting in highstrength fibers. Following these developments, PAN based commercial T300carbon fiber was introduced by the Toray Company of Japan in 1971 with atensile strength of 2.5 GPa. Through material and process optimization,by 1980, the tensile strength of T300 carbon fiber was improved to 3.5GPa. Currently, a widely used PAN based carbon fiber in aerospaceindustry, the IM7, has a tensile strength of 5.6 GPa and tensile modulusof 276 GPa. PAN based carbon fiber (T1100G) with a tensile strength of6.6 GPa and tensile modulus of 324 GPa was recently announced. Aftermore than 50 years of development, the tensile strength of high strengthPAN based carbon fibers is still less than 10% of the theoreticalstrength of the carbon-carbon bond, and about 30% of the theoreticalmodulus. On the other hand, continuous pitch based carbon fibers can bemanufactured with modulus as high as 965 GPa (>90% of the theoreticalmodulus). However, these high modulus pitch based carbon fibers haverelatively low tensile strength (3.1 GPa), resulting from largegraphitic grain boundaries and relatively low inter-planar shearmodulus. Although it is possible to process PAN based carbon fibers withtensile modulus approaching 600 GPa (which is 57% of the theoreticalvalue for graphite) via high temperature carbonization, this hightensile modulus is achieved at the expense of tensile strength. Clearly,there is room and need to increase the strength of carbon fibers whilealso increasing their modulus.

Man-made polymeric fibers are processed by melt or solution spinning.While PAN can be melt spun, currently all commercial PAN fiberproduction is based on solution spinning. Gel spinning is used formaking high strength and high modulus polymeric fibers and can lead tofewer entanglements in the fiber than in the melt and conventionalsolution spun fiber of the same molecular weight. Through gel spinning,high molecular weight polymer solutions can be spun with highermolecular alignment than what can be achieved by conventional solutionspinning, resulting in higher modulus of the carbonized fiber. However,existing methods of carbonization of gel spun PAN fibers have beencarried out as a batch process, and there are no reports of gel spunfiber carbonization in a continuous process.

There is a need for a carbon fiber that has both a high tensile strengthand a high tensile modulus.

Therefore, there is also a need for a method for continuouscarbonization of gel spun PAN fibers.

SUMMARY OF THE INVENTION

The disadvantages of the prior art are overcome by the present inventionwhich, in one aspect, is a carbon fiber that has a fiber tensilestrength in a range of 5.5 GPa to 5.83 GPa. The carbon fiber has a fibertensile modulus in a range of 350 GPa to 375 GPa. The carbon fiber alsohas an effective diameter in a range of 5.1 μm to 5.2 μm.

In another aspect, the invention is a tow that includes a plurality ofcarbon fibers. The carbon fibers have an average fiber tensile strengthin a range of 5.5 GPa to 5.83 GPa. The carbon fibers have an averagefiber tensile modulus in a range of 350 GPa to 375 GPa. The carbonfibers also have an average effective diameter in a range of 5.1 μm to5.2 μm.

In yet another aspect, the invention is a method of making a carbonfiber, in which PAN (poly(acrylonitrile-co methacrylic acid)) isdissolved into a solvent to form a PAN solution. The PAN solution isextruded through a spinneret, thereby generating at least one precursorfiber. The precursor fiber is passed through a cold gelation medium,thereby causing the precursor fiber to gel. The precursor fiber is drawnto a predetermined draw ratio. The precursor fiber is continuouslystabilized to form a stabilized fiber. The stabilized fiber iscontinuously carbonized thereby generating the carbon fiber. The carbonfiber is wound onto a spool.

These and other aspects of the invention will become apparent from thefollowing description of the preferred embodiments taken in conjunctionwith the following drawings. As would be obvious to one skilled in theart, many variations and modifications of the invention may be effectedwithout departing from the spirit and scope of the novel concepts of thedisclosure.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS

FIG. 1 is a flow chart showing one method of making a carbon fiber.

FIG. 2 is a schematic diagram of a PAN gel spinning apparatus.

FIG. 3 is a schematic diagram of a fiber drawing line.

FIG. 4 is a schematic diagram of a continuous stabilization andcarbonization line.

FIG. 5 is a graph showing oxidative thermal degradation temperatures ofseveral different carbon fiber types.

FIG. 6 is a photograph of a spool of a precursor fiber tow next to aspool of carbonized fiber tow.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the invention is now described in detail.Referring to the drawings, like numbers indicate like parts throughoutthe views. Unless otherwise specifically indicated in the disclosurethat follows, the drawings are not necessarily drawn to scale. As usedin the description herein and throughout the claims, the following termstake the meanings explicitly associated herein, unless the contextclearly dictates otherwise: the meaning of “a,” “an,” and “the” includesplural reference, the meaning of “in” includes “in” and “on.”

One representative embodiment of the invention includes a carbon fiberderived from gel spun polyacrylonitrile (PAN) that has a fiber tensilestrength in a range of 5.5 GPa to 5.83 GPa, a fiber tensile modulus in arange of 350 GPa to 375 GPa, an effective diameter in a range of 5.1 μmto 5.2 μm, and an oxidative thermal degradation temperature of at least815° C. Such a fiber can be arranged in a fiber tow with as few as 100carbon fibers. The fiber tow can be woven into a fabric or can be heldin a polymer matrix to give strength to components in a variety ofapplications (e.g., aerospace components and structural elements).

As shown in FIG. 1, one embodiment of a method 100 of making such acarbon fiber includes the steps of: grinding and dissolving PAN into asolvent 110; spinning the solution to form a precursor fiber and thengelling the precursor fiber in cold methanol 112; drawing the precursorfiber to a predetermined draw ratio 114; continuously stabilizing theprecursor fiber in a stabilization oven to form a stabilized fiber 116;and continuously carbonizing the stabilized fiber to form a carbonizedfiber 118.

In one experimental embodiment, as shown in FIG. 2, the PAN polymer usedwas poly(acrylonitrile-co-methacrylic acid) (PAN-co-MAA) copolymer,where the co polymer is 4.7 weight % MAA, with a viscosity averagemolecular weight of 513,000 g/mol (available from Japan Exlan Company,Osaka, Japan). The polymer was dissolved in N,N-dimethylformamide (DMF)at 90° C. at a concentration of 10.5 g/dl. In the experimentalembodiment, the prepared/degassed PAN solution was transferred topre-heated solution tank 212 at 75° C. at the spinning machine 210. Thesolution was stored in the tank 212 at 75° C. for 2 hours beforespinning and was pressurized with nitrogen gas to 200 psi. Thetemperature profile for solution tank, metering pump block, andspin-pack jacket was 75° C., 75° C., and 85° C., respectively

The prepared solution was spun using pump 214 that pumped the solutionthrough a spin pack 216 equipped with a 100-hole spinneret (holediameter of 200 μm). The spin-pack included a filter screen andspinneret. The solution filter screen employed a 20 μm stainless steelmesh filter. The spinneret included 100 holes with hole diameter of 200μm and L/D of 6. The metering pump had a capacity of 3.2 cc/rev. In theinitial stage of spinning, the metering pump was set at 50 rpm (the flowrate was 1.6 cc/min/hole). When the stable jetting of solution wasobtained, the metering pump was set to 16.8 rpm (the flow rate was 0.5cc/min/hole). The linear jet speed was 16 m/min.

The fiber 218 was spun into a gelation medium that included a methanolbath 220 maintained at −50° C. using an air gap between the surface ofspinneret and gelation medium of about 2 cm (e.g., in one experiment, anair gap of 19 mm was used). The gelation medium 220 was substantially100% methanol at −50° C. The gelation medium was circulated at a flowrate of 27 L/min.

The as-spun draw ratio was 3 and the post-spin draw ratio of theresulting fiber 224 was 8.2 (total draw ratio was 24.6). The fiber 224was then rolled on a winder 226. The precursor fiber 224 was taken up at48 m/min. Therefore, the spin draw ratio was 3.

As shown in FIG. 3, post-spin drawing was conducted with a multi-stepdrawing machine 300 at the drawing temperature as high as 180° C.Initially, the stored precursor fiber was mounted on the unwinding standof drawing line. While unwinding, one-third of the fiber spool wasdipped in the chilled methanol (−30° C.) 308 and the unwinding speed wasset at 2 m/min. The unwound fiber tow was passed through dancing roller310 and guide roller 311 for controlling unwinding tension at 30 gf. Atthe guide roller 311, continuous stream of chilled methanol 311 wasdripped onto the fiber tow to wash residual solvent (DMF).

The fiber tow was then wrapped three times at a godet 314 having rollersrun at 2 m/min at room temperature. (There was no drawing betweenunwinder and godet 314). Then, the fiber tow was transferred to godet315 where it was wrapped four times. The godet 315 rollers ran at 2.2m/min at room temperature (resulting in a draw ratio between godet 314and godet 315 of 1.1). Then, the fiber tow was passed throughspin-finish application bath 316.

The fiber tow was then wrapped 15 times at godet 317 having rollers thatran at 5.0 m/min at 110° C. (resulting in draw ratio between godet 315and godet 317 of 2.27). The fiber tow was then wrapped four times atgodet 318, having rollers that ran at 16.2 m/min at 180° C. (resultingin a draw ratio between godet 317 and godet 318 of 3.24). The fiber towwas then passed through spin-finish application bath 320 for a secondspin-finish. The fiber tow was then wrapped 20 times at a dryer roller322, which ran at 16.3 m/min (resulting in a draw ratio between godet318 and dryer roller 322 of 1.01). The dryer rollers 322 were in theisothermal chamber where the air temperature was controlled at 110° C.).The dried fiber tow was wound on a spool 324 and the winding tension wascontrolled at 35 gf. A spool of precursor fiber next to a spool ofcarbonized fiber is shown in FIG. 6.

The fiber tow was then stabilized and carbonized. As shown in FIG. 4, inthe stabilization and carbonization unit 400, the drawn fiber wasmounted on the unwinding stand 410 at continuous stabilization andcarbonization line and the unwound fiber tow 412 was conveyed to thefirst set of rollers 414 (each set of rollers is composed of fiverollers). The unwinding tension was controlled at 10 MPa based on theprecursor fiber diameter (11 μm). Then, the fiber tow was passed throughsix different stabilization ovens 416 with four different tensioncontrol zones. The detailed processing conditions are listed in thefollowing table:

Residence Temp Strain Time Zone (° C.) (%) (min) 1 180 6.0 20-44 2 1905.6 20-44 3 200 5.6 20-44 4 210 20-44 5 230 4.9 20-44 6 250 20-44 total24.0 120-264

Then, the stabilized fiber tow was passed through low temperaturecarbonization furnace 418, which included three separate temperaturecontrol zones. Strain was controlled using set of rollers before andafter the furnace. Nitrogen gas was purged on both ends of processingtube to maintain the inert environment. The detailed processingconditions are listed as follows: Zone 1 Temp=500° C.; Zone 2 Temp=600°C.; Zone 3 Temp=675° C.; Strain (%)=20; and Residence Time (min)=1-10.

Then, the low temperature carbonized fiber tow was passed through hightemperature carbonization furnace 420. The high temperaturecarbonization furnace has four separate temperature control zones.Strain was controlled using set of rollers before and after the furnace.Nitrogen gas was purged on both ends of processing tube and in themiddle of processing tube to maintain the inert environment. Thedetailed processing conditions are as follows: Zone 1 Temp=1450° C.;Zone 2 Temp=1450° C.; Zone 3 Temp=1450° C.; Strain (%)=−2 to −4; andResidence Time (min)=1-10. The carbonized fiber tow was then wound on 3″diameter polypropylene tube at a constant winding tension of 25 gf atwinder 422.

Using this method results in carbon fibers that are without any surfacetreatment and that are without any sizing.

In one experimental embodiment, the tensile properties and structuralparameters of the precursor fiber are listed in the table below. It isnoted that the tensile modulus of this gel spun PAN precursor is 20.7GPa. This value is significantly higher than the modulus value achievedin the solution spun PAN fiber, which is typically in the range of 7 to14 GPa. Higher tensile modulus in the gel spun fiber is a result ofsignificantly higher draw ratios achieved in gel spinning than insolution spinning.

PAN precursor fiber Effective diameter (μm) 11.0 ± 0.8  Density (g/cm³)1.207 Tensile Strength (GPa) 1.0 ± 0.1 properties^(a) Modulus (GPa) 20.7± 1.1  Elongation at break (%) 9.4 ± 1.5 WAXD d-spacing_((200,110)) (nm)0.527 analysis Crystallinity (%) 64 Crystal size L_((200,110)) (nm) 15.7FWHM_(azi, (200,110)) (degrees) 6.7

Using this method, carbon fibers have been produced in multiple gelspinning runs of precursor fibers and carbonization with average tensilestrength values in the range of 5.5 to 5.8 GPa and tensile modulus inthe range of 354 to 375 GPa. There are no other fibers with thiscombination of tensile strength, modulus and fiber diameter. Forexample, the tensile strength of current PAN based commercial fiber(M40JB) with a modulus of 377 GPa is only 4.4 GPa. However, the highertensile modulus in the inventive gel spun PAN based carbon fiber ascompared to the IM7 carbon fiber is without any loss in tensilestrength.

The degradation temperatures of various carbon fibers are shown in thegraph in FIG. 5. The following table compares characteristics ofexisting carbon fibers to those of one experimental embodiment of thecarbon fiber of the present invention:

PAN based carbon fibers carbon fiber of Pitch-based the present carbonfiber invntion IM7 T300 K-1100 Cross-sectional area (μm)² 19.6-21.2 21.238.5 113.1 Density (g/cm³) 1.77-1.79 1.78 1.76 2.20 Tensile Strength(GPa) 5.5-5.8 5.6 3.6 3.1 properties Modulus (GPa) 354-375 276 230 965Oxidative thermal degradation 815 714 771 877 temperature (° C.) WAXDd₀₀₂ (nm) 0.344 0.348 0.349 0.337 analysis* L₀₀₂ (nm) 1.9 1.6 1.4 26.6L₁₀ (nm) 2.5 2.1 2.0 21.1 FWHM_(azi,002) 23.1 30.3 35.0 0.5-1.0(degrees) Raman I_(G)/I_(D) 0.46 0.43 0.39 2.00 intensity ratio *d₀₀₂,L₀₀₂, and L₁₀ are inter-planar graphitic spacing, crystal sizeperpendicular to (002), and crystal size within graphitic plane alongthe fiber axis, respectively.

The above described embodiments, while including the preferredembodiment and the best mode of the invention known to the inventor atthe time of filing, are given as illustrative examples only. It will bereadily appreciated that many deviations may be made from the specificembodiments disclosed in this specification without departing from thespirit and scope of the invention. Accordingly, the scope of theinvention is to be determined by the claims below rather than beinglimited to the specifically described embodiments above.

What is claimed is:
 1. A carbon fiber characterized as having a fibertensile strength in a range of 5.5 GPa to 5.83 GPa, a fiber tensilemodulus in a range of 350 GPa to 375 GPa, and an effective diameter in arange of 5.1 μm to 5.2 μm.
 2. The carbon fiber of claim 1 having anoxidative thermal degradation temperature of at least 815° C.
 3. Thecarbon fiber of claim 1 arranged in a tow of no more than 100 carbonfibers.
 4. A tow, comprising a plurality of carbon fibers that have anaverage fiber tensile strength in a range of 5.5 GPa to 5.83 GPa, anaverage fiber tensile modulus in a range of 350 GPa to 375 GPa, and anaverage effective diameter in a range of 5.1 μm to 5.2 μm.
 5. The tow ofclaim 4, wherein the carbon fibers have an average oxidative thermaldegradation temperature of at least 815° C.
 6. The tow of claim 4,wherein the plurality of carbon fibers includes about 100 carbon fibers.7. The tow of claim 4, wherein the plurality of carbon fibers arewithout any surface treatment and are without any sizing.
 8. A method ofmaking a carbon fiber, comprising the steps of: (a) dissolving PAN(poly(acrylonitrile-co methacrylic acid)) into a solvent to form a PANsolution; (b) extruding the PAN solution through a spinneret, therebygenerating at least one precursor fiber; (c) passing the precursor fiberthrough a cold gelation medium, thereby causing the precursor fiber togel; (d) drawing the precursor fiber to a predetermined draw ratio; (e)continuously stabilizing the precursor fiber to form a stabilized fiber;(f) continuously carbonizing the stabilized fiber thereby generating thecarbon fiber; and (g) winding the carbon fiber onto a spool.
 9. A carbonfiber made according to the method recited in claim
 8. 10. The method ofclaim 8, wherein the carbon fiber is made without any surface treatmentand without any sizing.
 11. The method of claim 8, wherein the gelationmedium comprises methanol.
 12. The method of claim 8, wherein thegelation medium has a temperature of about −50° C.
 13. The method ofclaim 8, wherein there is an air gap of about 2 cm between the spinneretand the top surface of the gelation medium.
 14. The method of claim 8,wherein stabilizing step comprises passing the precursor fiber through aplurality of stabilization zones, including a first stabilization zonethat heats the precursor fiber to a temperature of about 180° C. and alast stabilization zone that heats the precursor fiber to a temperatureof about 250° C.
 15. The method of claim 14, wherein the plurality ofstabilization zones includes: a first stabilization zone that heats thefiber to about 180° C.; a second stabilization zone that heats theprecursor fiber to about 190° C.; a third stabilization zone that heatsthe precursor fiber to about 200° C.; a fourth stabilization zone thatheats the fiber to about 210° C.; a fifth stabilization zone that heatsthe precursor fiber to about 230° C.; and a sixth stabilization zonethat heats the precursor fiber to about 250° C.
 16. The method of claim8, wherein carbonizing step comprises passing the precursor fiberthrough a plurality of carbonization zones that heat the fiber to atemperature of about 1450° C.
 17. The method of claim 16, wherein theplurality of carbonization zones includes: a warming zone that heats theprecursor fiber to about 500° C.; a first carbonization zone that heatsthe precursor fiber to about 600° C.; a second carbonization zone thatheats the precursor fiber to about 675° C.; and a third carbonizationzone that heats the precursor fiber to about 1450° C.
 18. The method ofclaim 8, wherein the carbon fiber has a fiber tensile strength in arange of 5.5 GPa to 5.83 GPa, a fiber tensile modulus in a range of 350GPa to 375 GPa, and an effective diameter in a range of 5.1 μm to 5.2μm.
 19. The method of claim 8, wherein the carbon fiber has an oxidativethermal degradation temperature of at least 815° C.
 20. The method ofclaim 8, further comprising the step of bundling a plurality of carbonfibers into a tow of about 100 carbon fibers.